Function names are lowercase, with words separated by underscores. Descriptive names are encouraged. Function names typically evoke operations applied to arguments by the interpreter (e.g., print , add , square ) or the name of the quantity that results (e.g., max , abs , sum ). Parameter names are lowercase, with words separated by underscores. Single-word names are preferred. Parameter names should evoke the role of the parameter in the function, not just the kind of argument that is allowed. Single letter parameter names are acceptable when their role is obvious, but avoid "l" (lowercase ell), "O" (capital oh), or "I" (capital i) to avoid confusion with numerals.